citric acid, an aliphatic hydroxy acid, has been known as a natural fruit acid since the last century.
In nature, it is widely present as the acid and in the form of its salts and can be found in fruit, vegetables, milk and meat.
It is widely used in foods and beverages, in the pharmaceutical industry and for purification process and as an antioxidant so that commercial preparation independent of natural sources is necessary.
Citric acid has been long made from sugar or molasses by a fermentation process utilizing molds of the genus Aspergillus by the surface process and since the middle of this century by a submerged process.
Both processes are batch operations which can achieve yields between 57 to 77% within 1 to 2 weeks.
Since 1960, predominantly in Japan, citric acid has been produced utilizing other microorganisms, especially of the genus Candida, on other substrates of which n-alkanes are the most prominant.
In the Japanese Patent Publication J-58-138387, a process for producing citric acid from n-alkanes is described in which yeast strains of Candida or Saccharomycopsis are used to carry out fermentation in an agitated oxygen-aerated two-phase system where the optimum conversion is achieved with oxygen saturation values of 10 to 45%. The citric acid which is formed is isolated after interruption of the agitation by a phase separation from the aqueous phase which can be replenished a number of times. In this manner, citric acid yields up to 44 g/l can be obtained. Particulars with respect to the formation of isocitric acid are not available. A commercial production by this process is unknown.
In recent years, investigations into citric acid production has concentrated on the use of sugar containing media.
In the East German Patent Document 248,376, for example, citric acid is recovered in a batch process in which initially yeast (Yarrowinia lipolytica) is cultured upon a nutrient medium containing n-alkanes or substances with long CH.sub.2 chains with addition of the customary mineral and trace element salt solutions until the N source has been consumed, sugar (125 g/l glucose or invert sugar) being Added and being transformed 90% to citric acid with 5% to isocitric avid. In this batch process significant problems arise with respect to the handling of the residual solution.
A continuous citrate production which would be favorable on economic grounds has not been industrially performed heretofore; on the contrary doubts about the utility of such a process have been repeatedly expressed (H. J. Peppler et. al. "Microbial Technology" Vol. I, Page 363, Acad. Press., London (1979); P. E. Milsom, "Food Biotechnology" Vol. I (1987) Page 291).
T. K. Klasson et al. (Appl. Biochem. Biotechnol. Vol. 20/21 (1989) Pages 491-509) have reported on comparative tests with continuous and bathwise production of citric acid by fermentation with yeast (Candida lipolytica). The continuous fermentation is carried in a single fermentor with a maximum residence time of 30 hours and &gt;50% air saturation with glucose content (in the feed) of 30 to 150 g/l with variable ammonium concentrations from 0.3 to 1.3 g/l NH.sub.4 Cl. With the continuous fermentation, fairly specific production rates from 0 to 0.11 g/gh are obtained with significant isocitric acid formation (3 to 6.5g citric acid/g isocitric acid). In the batch process up to 40 g/l citric acid with isocitric acid is formed while in continuous operation the acid concentration runs up to 14 g/l.
The results given by Klasson et al are not promising for a commercial production since both the total acid concentration and the citrate/isocitrate proportions are unsatisfactory.